The subject of the present invention is a mixer-ejector with jet effect and the invention can be applied in particular to the energy converters intended for the recompression of gas or wet steam.
These converters, known as "thermocompressors", are dilution devices in which an exchange of energy is produced between a driving gas (or steam) and a gas (or steam) which is drawn in.
Thermocompressors, being static devices, are very simple and reliable. They are employed particularly in evaporators in the field of agriculture and food processing for concentrating an aqueous solution and/or suspension (sugar manufacture, dairy products, distilleries, sea water desalination plants, and the like).
Originally, the use of ejectors was restricted to the construction of simple devices with a low entrainment ratio (relationship of the induced flow drawn in to the motive or inductor flow) at a low recompression ratio (relationship of the delivery pressure to the suction pressure).
In general, an ejector incorporates a motive nozzle which opens, delivering a flow Q' of steam or gas at a total pressure P' and a total temperature T', into a steam or gas suction sleeve (induced flow Q", total pressure P" at the total temperature T"). This sleeve is extended by a mixer in which the energy exchange between the two flows takes place and then by a diffuser which converts the resultant kinetic energy of the mixture with a flow Q, into a static pressure P at the total temperature T.
An improvement in the ejectors has been to replace the single inductor by a plurality of nozzles which, by virtue of the division of the driving jet, improves the quality of the mixing and makes it possible to increase the entrainment ratio for a given recompression ratio.
To increase the capacities of these ejectors, that is to say to increase the recompression ratio P/P" it has been necessary to generate a supersonic mixed flow at the inlet of the mixer. As it changes to a subsonic velocity, this flow produces a shock wave. The location of this change must be chosen judiciously to avoid a loss in efficiency and even a deterioration of the equipment (ejector and even downstream receiver).
It has thus been proposed to insert a venturi between the mixer and the diffuser and to create conditions for the change from supersonic to subsonic flow to take place opposite the venturi throat.
For optimum throat dimensions (sonic throat), this arrangement makes it possible to ensure a continuous recompression without flow shock. However, this type of device presents two difficulties:
the maximum efficiency of the device corresponds to a throat diameter below the critical priming size; and
the operating conditions can change with time as a function of fouling or of production requirements. The device must therefore have a fairly wide range of adaptability.
To overcome these disadvantages, a variable geometry is produced, which comprises all or a part of the converger, the throat at the outlet of this converger and at least a part of the diverger.
Under these conditions the initial priming is produced at a throat size greater than or equal to the critical size, the throat is then closed again until there is obtained the required pressure level permitting the change from supersonic flow (in the converger) to subsonic flow (in the diverger) without shock wave formation and with the losses reduced to a minimum.
The variable-profile part consists advantageously of a sleeve or flange made of a distortable eleastic material which is acted upon by a pressurized fluid driven by the upstream pressure, the suction pressure or the pressure downstream of the ejector. The distortion of the elastic wall adapts the throat to the conditions of use.
We have found, however, that the use of this external pressure to distort the elastic sleeves gives rise, if no special precaution is taken, to a creased inner cross-section with a multilobe appearance (see FIGS. 1 and 2) with more or less pronounced lobes, which is incompatible with a homogeneous aerodynamic flow.